1. Field of the Invention
The invention generally relates to thin films that exhibit second-order nonlinear optical (NLO) properties. In particular, the invention provides materials and methods for producing such films by forming, on a substrate, alternate layers of a polymer and a low molecular weight chromophore.
2. Background of the Invention
Materials that exhibit second-order nonlinear optical (NLO) properties are key components in electrooptic modulators and frequency-doubling devices. [1] Second-harmonic generation (SHG), in which incident light at one frequency is converted into light at twice that frequency, is one example of second-order NLO phenomena and is often used as an experimental probe of the second-order susceptibility χ(2). A material must have a non-centrosymmetric structure to possess a nonzero χ(2). Electrooptic modulators have traditionally employed ferroelectric inorganic crystals, such as lithium niobate or potassium dihydrogen phosphate, which are formed at high temperatures. However, organic NLO materials offer several advantages in performance, such as higher nonlinear susceptibilities, higher modulation rates, and potentially lower device fabrication costs. [2] Organic films exhibiting nonzero χ(2) values have been fabricated using a variety of methods, including electric field poling, [3] Langmuir-Blodgett (LB) films, [4] and covalent self-assembly. [5] Both poled polymer systems and LB films have been made with non-centrosymmetric structures that exhibit relatively high values for χ(2), but poor temporal or mechanical stability restrict their potential applications. [6] Deposition processes using reactive silane compounds require organic solvents and high temperatures. [5]
There is a large and growing body of literature on the use of layer-by-layer (LBL) methods for fabricating nanostructured films for a variety of applications. The LBL technique, which relies on purely electrostatic interactions, was first developed by Iler [7] and further elaborated upon by Decher et al. [8] Several research groups have demonstrated that the NLO films made by this technique have greater thermal and temporal stability than poled polymer systems. [9] A related approach that could be employed to fabricate NLO materials involves the use of low-molecular-weight dye molecules and polymers as film constituents. Yamada et al. made films of poly(diallyldimethylammonium chloride) and Erichrome Black T that exhibited an SHG intensity that increased only for the first five bilayers and then reached a plateau. [10] Koetse et al. [18] experimented with films grown with polyamines and reactive dyes but found that there was no polar order within the film layers. The authors indicate that the SHG signal they observed most likely originated from the dye molecules at the surface of the support. Other research groups have found that ionic interactions alone are not sufficient for constructing LBL films with low-molecular-weight chromophores. [11]
It would be of great benefit to have available stable NLO films with the large number of bilayers needed for electrooptic devices, and methods for their production. Further, it would be of benefit to have available a combination of low-molecular-weight chromophoric molecules and polymers that could be used to construct such films.